A070746 Numbers k such that k 1's followed by k is a prime.

1, 7, 709, 2203, 4481, 5107, 11489

Offset

1,2

Comments

The number corresponding to a(3) has been certified prime with Primo; those corresponding to a(4), a(5), a(6) are probable primes.

a(8) > 147000, if it exists. - Giovanni Resta, Jun 22 2018

Links

Table of n, a(n) for n=1..7.

Example

7 is in the sequence because 11111117 (seven 1's followed by 7) is prime.

Mathematica

Do[ If[ PrimeQ[ FromDigits[ Join[ IntegerDigits[(10^n - 1)/9], IntegerDigits[n]]]], Print[n]], {n, 1, 2000, 2}]

Prog

(WinPFGW) Primality testing r(5107)*10^len(5107)+5107 [N-1/N+1, Brillhart-Lehmer-Selfridge] r(5107)*10^len(5107)+5107 is Fermat and Lucas PRP! (79 seconds). - Jason Earls
(PARI) isok(n) = isprime(eval(concat(apply(x->Str(x), vector(n+1, j, if (j>n, n, 1)))))) \\ Michel Marcus, Jun 15 2018

Crossrefs

Cf. A068817, A084428.

Sequence in context: A129291 A229149 A159815 * A068004 A098110 A062334

Adjacent sequences: A070743 A070744 A070745 * A070747 A070748 A070749

Keyword

base,nonn,more

Author

Robert G. Wilson v, May 03 2002

Extensions

a(4) from Jason Earls, May 06 2002

a(5) from Rick L. Shepherd, Sep 02 2002

a(6) from Farideh Firoozbakht, Jun 29 2003

a(7) from Farideh Firoozbakht, Jul 04 2003

Edited by T. D. Noe, Oct 30 2008

Status

approved